Melchior Díaz was an early Spanish explorer of Western North America. In 1540 he journeyed up the river now known as the Colorado from where it drains into the Gulf of California and sent expeditions from the river as far as the present day Imperial Valley region of California (USA) and Baja California (Mexico). He was following in the footsteps of fellow explorer Hernando de Alarcón, who was exploring the Gulf of California at the same time.

His party made the first European crossing of the Colorado, near the modern-day city of Yuma, Arizona. They discovered and reported geothermal hot springs, probably the ones near Calexico, Mexico.